The present invention relates to a process useful for fabricating printed circuit boards and particularly for the metallization of plated through holes in printed circuit boards. Specifically, the invention relates to the use of a conducting polymer layer for electrolytic metallization of printed circuit boards which eliminates the necessity for precious metal seeds and electroless copper baths in the manufacture of printed circuit boards.
Prior art processes for the metallization of plated through holes have usually included the step of seeding or using a catalyst, usually a noble metal salt, within the plated through hole. The next step has been to immerse the seeded circuit board in an electroless bath, usually copper, in order to plate a thin layer of copper, approximately 2 to 5 microns thick, in the plated through hole. The thin copper layer renders the plated through holes sufficiently conductive so that a thicker copper layer may be added by conventional electrolytic deposition.
While the described process is currently used in manufacturing printed circuit boards there are several drawbacks to the process. First, by their very nature, electroless baths are unstable and require close monitoring and control. Electroless copper baths fluctuate between being too stable resulting in plated through hole voids and being too active which results in homogenous decomposition of the bath. Second, formaldehyde which is probably the most commonly used reducing agent today in electroless copper baths is toxic and poses difficulties in waste treatment and disposal. Third, precious metal seeds are expensive and often have a limited useful lifetime.
Currently there are several techniques emerging in the industry which eliminate the necessity for precious metal seeds and electroless copper baths for printed circuit board manufacturing. In one technique a plated through hole is coated using a conductive colloidal graphite suspension (Black Hole Process, Olin Hunt Specialty Products). Once coated, the plated through hole is plated electrolytically, thus eliminating the need for electroless metal deposition. The described process is only useful for the plated through holes and is not extendable to plating extended printed circuit board surfaces. Therefore, while the described process is of value for full panel plating by substractive processing, it is of limited use for pattern electroplating.
The present invention concerns an electrically conductive polymer coating which may be applied to a printed circuit board composite substrate. Preferably, the polymer coating is applied from a non-aqueous or aqueous solvent followed by an electroplating process that is performed in an aqueous solution. Moreover, the polymer coating is stable in air since the polymer may be cast, painted, spin-coated or dip-coated onto a substrate and may even be removed from the substrate to generate a pattern.
The polymer coating in practicing the present invention covers a predetermined region of the printed circuit board varying from covering the entire substrate surface including any through hole, blind holes or vias to just coating any through holes, blind holes or vias. The polymer is sufficiently conductive that the composite may be electrolytically plated simply by immersion into a copper ion solution, such as a copper sulfate bath. The polymer film precludes the need for both precious metal seeds and electroless copper deposition prior to electrolytic metallization. Moreover, pattern electroplating using a thin film of conducting polymer according to the teachings of the present invention permits enhanced circuit line profiles which is an important consideration in fine line circuitization.
Polymers useful for the electrolytic metallization of printed circuit boards include substituted and unsubstituted polyanilines, substituted polyparaphenylenevinylenes, substituted and unsubstituted polythiophenevinylene, substituted and unsubstituted polyfuranvinylene, substituted and unsubstituted polypyrrolevinylene, substituted and unsubstituted polythiophenes, polyazines, substituted polyfurans, substituted polyselenophenes, substituted and unsubstituted poly-p-phenylene sulfides and polyacetylenes formed from soluble precursors.
The above conducting polymers useful in practicing the present invention contain a partially conjugated .pi. system. A solution is formed of the polymer. To the solution is added a doping species (dopant precursor) which on exposure to energy generates a dopant which dopes the polymer to the conducting state. The addition of the dopant results in an expansion of the extent of the conjugated .pi. system in the individual polymer molecule. It is not necessary to extend the conjugated .pi. system over the full extent of the molecule. It is only necessary to sufficiently extend the .pi. conjugated system of an individual molecule so that after the solvent is removed the .pi. conjugate part of an individual molecule is adjacent to a part of the .pi. conjugated part of an adjacent molecule. In the .pi. conjugated system an electron is essentially delocalized over the entire .pi. conjugated bonds. These electrons are more loosely bond and are available for electrical conduction. When an electric field is applied, an electron can flow along an individual molecule and hop from one molecule to an adjacent molecule in a region where the .pi. conjugated parts of the adjacent molecules overlap.
The conductive properties of these polymers and their uses are described in co-pending U.S. patent application Ser. No. 07/357,565, field May 26, 1989, entitled "Electrically Conductive Polymeric Materials and Uses Thereof" and assigned to the same assignee as the present patent application, which co-pending application is incorporated herein by reference.
The preferred polymers for the electrolytic metallization of printed circuit boards is polyaniline. A major advantage for the use of the polymer polyaniline in fabricating printed circuit boards according to the present invention is the relatively low cost of producing the polymer.